Crystal diode for microwaves



Unite Stats CRYSTAL DIODE FGR MICROWAVES Hans Bertil Hdaird, Hagersten, Sweden, assignor to Telefonaktiebolaget L M Ericsson, Stockholm, Sweden, a company of Sweden The object of this invention is to provide a crystal diode for microwaves having a new construction, which is advantageous from the impedance point of view.

Crystal diodes are now generally used for mixing and detecting purposes in the microwave range. Such a diode consists of two electrodes, one of which carries a semiconducting element, usually a silicon crystal. A thin, bended metal wire is attached to the other electrode, the point of said wire being held against the semiconducting element. The length of the Wire is, however, usually not small in comparison to a quarter of a wavelength at the frequencies in the centimeter wave range. Such a wire has an inductance, which causes difficulties in the microwave range, and the value of said inductance ought to be as small as possible for optimum operation.

This invention relates to a crystal diode having the value of said inductance largely reduced. This makes it possible to use the diode with a good result at the highest frequencies of the microwave range.

The crystal diode according to the invention consists of a first electrode comprising a semiconducting element and a second electrode with a point, which is held against the semiconducting element under spring pressure. The diode is mainly characterized by the electrodes being cylindrical with a rather large diameter and by said other electrode being conical with a base diameter approximately equal to the diameter of the diameter of the cylindrical part of the electrode, which diameter is in the order of magnitude of 2-5 millimeters, so that the series inductance of the diode will be as small as possible.

The invention will be closer described in connection with the accompanying drawing, where Fig. 1 shows a crystal diode according to the invention, Figs. 2-4 show some different detail constructions, and Fig. 5 shows another embodiment of a crystal diode according to the invention.

Fig. 1 shows an embodiment of a diode according to the invention. 1 is a metallic electrode, which carries a semiconducting element 2. This may suitably be a silicon crystal, but other semiconducting materials having similar properties, e. g. germanium, may also be used. The

other electrode consists of a tubular, metallic cylinder 3, in which another metallic part 4 having a conical point is movable in the axial direction. The point of said part 4 is pressured against the semiconducting element by help of a coil spring 5. The pressure of the conical point against the semiconducting element may be ad justed by a screw 6, which is screwed into the tubular cylinder 3. In order to be able to maintain the pressure at a constant value the conical part 4 shall be able to slide easily in the cylinder 3. It would therefore be advantageous to have a lubricant between the cylinder 3 and the part 4. On the other hand a good electrical contact must exist between them. Therefore the lubricant ought to have a rather high conductivity, and may suitably consist of e. g. graphite. Another way of obtaining a good electrical contact between the cylinder 3 2,777,949 Patented Jan. 15, 1957 and the movable part 4 is as follows. The length c of the movable part 4 is made equal to an odd number of quarter wavelengths, e. g. wavelength, at the frequency, at which the diode is intended to be used. Direct electrical contact between the cylinder 3 and the end of said movable part 4, which end is most distant from the conical point, is prevented by a ring 10 of some nonconducting material, e. g. nylon. This open circuit is then transformed by the quarter wavelength 0 into a short-circuit between the cylinder 3 and the portion of the movable part 4, where the conicity begins. A ring of some non-conducting material, which ring is similar to the previously mentioned ring 10 may be inserted also round that portion. In these cases the direct current will pass from the movable part 4 through the coil spring 5 and the screw 6 to the cylinder 3. In order to get the diode rather unsensitive to mechanical shocks the movable mass, thus mainly the part 4- ought to be made 1igi1t.- This part may suitably be made hollow, as is shown by the figure, and ot' a light material. The point pressured against the semi-conducting element ought, however, to be made of some hard material, e. g. tungsten. A suitable construction of the movable part 4 is shown by Fig. 2. The conical point is provided with a hole in the axial direction, and a thin, straight metal wire 7 has been inserted into that hole. This construction gives the advantage of the wire '7, which is to be pressured against the semi-conducting element, being able to be made of a suitable, hard material, e. g. tungsten, while the rest of the movable part may be made of some light metallic material. The wire 7 may suitably first be pressed into the hole and then sharpened, so that a suitably formed point will be obtained. The length a of the portion of the wire, exceeding outside the point of the movable part 4 ought to be considerably less than a quarter of a wavelength. That condition ought not to be ditiicult to fulfill by this construction. The length 12 of the con ical portion of the part 4 shall also be short in comparison to half a wavelength and shall preferably not exceed wavelength. Whether the conical point of the part 4 comprises a metal wire 7 or not, the device makes it possible to obtain a very low inductance in series with the resistance of the barrier layer. Because also the capacitance shunting the resistance of the barrier layer ought to get a small value in he construction according to the invention the diode ought to be useful down to the shortest Wavelengths in the microwave range.

It may be desirable at the manufacture of crystal diodes to have a possibility of adiusting the location of the point pressured against the semi-conducting element in order to find a suitable contact point. This may be efiected in a very simple manner. The point pressuring against the semi-conducting element may e. g. be placed a little eccentrically. By providing the movabie part 4 with two diametrically situated grooves 53 it will be pos sible easily to change the location of the contact point between the point of the electrode and the semi-conducting element by help of a screwdriver. Another way of obtaining the same result is as follows: The semi-conducting element 2 is fastened at the end of a screw ii. If either the conical point of the previously mentioned part 4 or the semi-conducting element 2 is placed eccentrically, the contact point between them may be easily varied by turning said screw 11. Said variation of the contact point may in this case be etlected, although the spring 5 has been previously inserted.

The electrode 1, which carries the semi-conducting element, need not necessarily be constructed as in Fig. 1. In Fig. 3 there is shown another embodiment of said electrode. The construction imports an inductance being obtained in series with the capacitance of the barrier 'ni'iid t layer. The value arsaitijtiifihr'iic' '1; determined by the geometrical dimensions";- and at the manufacture of the crystal I diode said inductance may ea sily he given a f ra e i i it t he adv l' ez enhish .ififibtamed by the curved "contact wire of crystal diodes aeporaing to h'revious. constru'btions these preyious ciinstrlictions it difficult toobtain equal values the iiiductairice at different diodes of cne and the in {construction cc ording tofig. 3 said 1 uc aiice ,shall suita y be given value that thc capacita'iice oi the earn r layenis come certaih desired frequency range T sha case if the electrode l is cohstructed acc dingto EigA. Th? t 'Q' b' '1 d .3. 2t d q eer held. to getter a non coiidu ctingi material Q e g glass, ceramic or' tlielike. I Thes ac ween onducting mac' a th i s wd .6. ha emir or uctinaelem 'j s i fi d' wit w X 9r Sam athe 1 9E399.- diictiii 'g'i'hateria'l having small losses and a small dielectric constant. a I V W It ay: a asl lj l m eteet q ele rod s. 1 and 3 with somewhat diflerent diameters, so that the diode maybe easily placed e. g. iiiawaveguide Fig.' 5 shows another embodiment o f acrystal diode according to the invention. .1 is a first electrode of a conducting material, which electrode carries a semi-conducting element 2. The point of a thin metal wire 7 is hfeld against that semi-conducting element. v S aid wire belongs to the other electrode, thefiiked part of which is indicated by 3. The end or the fixed par H the semi-conducting element, is conical and hasan axial or excentrical hole, in which said thin metal wire movable. The upper portion of this wire is bent or constructed in another manner, so that the p oint of the wire will be heldag'ainst the semi-conducting" element under spring pressure. The electrode li' and. the l ixed pare 3 of the other electrode are held together by non-conducting material 9, e. g. glass, ceramic or the like. The space between this non-conducting material the two elec trodes and the semi-conducting element may 'suitably be filled with wax or some other. non-conducting material having small losses and a small diaelectric constant. I

The upper end of the thinwirefis fastened to the fixed part 3 of said other electrode insu'ch aimanner, that a good electrical and mechanical contact will beobtaihed. The direct current through the diode will then passthis fastening point. Highfrequ'ency currentsshall, however, be able to pass from the thin wire 7 dire ctly to the con'ical end of the fixed part 3 of said other electrode. This may be efiected in two ditferent manners, 7 surfaces between the thin wire 7 and the hole, in which the wire is movable, are coated with a lubricant with a good electrical conductivity, e. g. g'raphite, ,or's'aid sliding surfaces may also be coated with an insulating material,

so that a direct Contact between th" hole and the wire prevented. In the lattpcase the length c (moreexactly the electricallength). of said hole shall be chosen so as to 'b e'an odd numberof qiiarter Wavelengths, e; g. wavelength, at thefrequ'ency at which the diode is intended to be used. A Because the upper end of said hole is open, the lower end of said hole will act as if it were shortcir cuit'ed from the high frequency point of view. In order to make the ubper end of said hole act exactly as an open circuit, the len'gth d (i. e. more exactly the electrical lengthy of lthe cayity 12 above said hole shall bean o'ddnuniber of quarter wavelengths at the frequency, atwhich the' diode is" intended to be used. I I M A In order to make the location of the cohtact' pdiirtbetween the point of the wire 7 and thesem'i conducting element 2 adjustable, the point of saidwire and the semiconducting element may also-in this construction be located excentrically in relation-to'each' other. According tothe figure the 'serni=conductin"element' recanted by a screw 11, by means of which the semi-conducting elesame sort er area t 3 which faces Either the sliding metal surfaces of the 'iiiefit b'l, 53 that the location or as amtact point may be adjusted.

The length a of the portion of the wire 7, which portion exceeds the conical part of said other electrode ought to be small and preferablyicoris'ideiably less than a quarter of the smallest wavelength, at which the diode is intended to be used.

ffhe length bet the conical portion of saidl'othr ,ele'b:

trode" part 3. also .ou'ght tobe small iandig'refeiably less than one eighth of the smallest wavelength; afwliiclithe diode is intended to be used. r r n The crystal diode may of 'coiirs'e also in this embodiment; be constructed according to Fig, 3 or 4 so that the electrode, whichcarries the semiconducting element, is constructed to form a-reaerance of'siich a value and sign, that the capacitance of the barrier layer will be mwh'in sily inset-table into a wavesaidTse'rni-c nduCting a ZLA crystal diode acceding t r 7 portion of the wire p rot ru cling" m said her" is' of a length considerably less thaiifo v the 'shortestlv v avelnth With which aid diod i H y A crystal diode ac d ingtoiclm 1,: i1 ele trgcany'cotiducnv Iiihrtcating 'if teriat between Said'metaI wire? and -the wall de'fi through" the respectiv i'lectrode' thereb mechanical friction and raw e lect'ridisist the metal wire andfthesaid"wall. v y

' 4. A crystal d'iode accdifding 'to 1 said lubricating material cbrripfris'es graphlte. H s. Ac'rysta'l diq eac'c'ordingto alarm" l, whetfin bore through s aid conical member has ari" e lct,r ic lly effective length of an odd number of'qua 'i" wavelengths, and wherein the 'wire 'portion withinfstaid iore' is'elc trically insulated from thesurrdiindin wall of' 'the 'cbificdl member. l 6. A crystal diode according to claim i hfm the said movable wire i'scoated Withinsulatibiiniate al. 7. A crystal diodeaccordingto claim 1, urer said electrodesare 'mouhted axially rotatable r'el'atl each other, and wherein said"p'0'int' of contact"be said wire and said semrcondn'c'ting 'me'inber ccen'tr'ic relative'to the rotatibnal axis of at' leas'fo'ne of s'l'a'id electrodes for varying the location of said contac't'tioint on the semi-conductingmember by rotatingshia'eteetrbaes relative to each other. 3 I

8.' A crystal diode according 'to claim "7, whreiii"the electrode including said semiconducting mniberfis"a iially rotatable and' said contact point is'e'cce'ntricrela't'ive to the rotational axis'of thesaid electrode.

9. A crystal diode according'to cia'ini 1,wlie'rein the said electrodes have diamete rs"diife"rent' 'onefrom the" other so'as to'facilitate insertion of 'the'diolie iii a wave guide. I

10. A crystal diode according'to claim l,"wher'e'in" a sleeve member made of'electricall'y noii cor'iduc'ting ma": teriali-sdisposedbetweensaid'two elec":odesfor 'jo' the"'sar'ne',' the said protruding wire 'fiortion"critiidm through said sleeve member.

11. A crystal diode according to claim 10, wherein the space bounded by said sleeve member and said electrodes is filled with a non-conductive material having a low dielectric constant. 1

12. A crystal diode according to claim 1, wherein the said electrode including the non-conductive member is of a configuration such as to form a reactance having a value and sign compensating for the capacitance of the barrier layer of the diode within a predetermined range of frequency.

References Cited in the file of this patent UNITED STATES PATENTS Carl-son May 21, Lundquist l Mar. 29, Southworth Jan. 3, Ohl May 10, Brittain June 14, Matare May 8, 

